Platelets, like a number of other secretory cells, display a striking shift in phosphoinositide (PI) metabolism when challenged with agonists such as thrombin, ADP, epinephrine, or collagen. The role played by PI in the secretory process has yet to be elucidated. However, perturbations in PI turnover are so consistently associated with stimulus-secretion that closer study of PI metabolism in platelets is warranted . Defects in PI metabolism may underlie certain abnormalities in human hemostasis and thrombosis. To dissect the various aspects of PI metabolism, we propose to examine: 1) endogenous PI-phosophodiesterase activity; 2) labeled glycerol incorporation into PI of platelets undergoing secretion in response to thrombin. In related studies, we intend to analyze; 3) the amount of PI at the exterior of the platelet membrane; 4) the branch-point enzymes controlling PI synthesis which may display altered activity during secretion; 5) the extent of synthesis of myoinositol by human platelets; 6) whether an association exists between PI and a platelet membrane protein component, and whether it is alterable by exposure of platelets to thrombin. In conjunction with this work, we expect to examine platelets displaying defective secretion from eight patients with recognized functional defects.